Digital lighting technologies, i.e., illumination based on semiconductor light sources, such as light-emitting diodes (LEDs), offer a viable alternative to traditional fluorescent, HID, and incandescent lamps. Functional advantages and benefits of LEDs include high energy conversion and optical efficiency, durability, lower operating costs, and many others. Recent advances in LED technology have provided efficient and robust full-spectrum lighting sources that enable a variety of lighting effects in many applications. Some of the fixtures embodying these sources feature a lighting module, including one or more LEDs capable of producing different colors, e.g., red, green, and blue, as well as a processor for independently controlling the output of the LEDs in order to generate a variety of colors and color-changing lighting effects, for example, as discussed in detail in U.S. Pat. Nos. 6,016,038 and 6,211,626.
To keep pace with rapidly advancing LED technology, LED driver circuitry (“LED drivers”) have been designed and re-designed to supply suitable drive currents and drive voltages for delivering desired light outputs from a wide variety of LEDs. The LED driver designs, however, lead to a potentially unmanageable increase in the number of different types of commercially available LED drivers. Conventional LED drivers incorporate control means, such as external variable resistors, that enable control of the output current of the LED drive, while keeping the maximum voltage fixed. With this approach, the full power capability of the LED driver is not fully utilized.
FIG. 1 is graph illustrating output current and voltage of conventional drivers. As mentioned above, a conventional LED driver has a fixed maximum output voltage limit, which dictates the operating area of the LED driver. For example, referring to FIG. 1, operating area 110 corresponds to a first (530 mA, 150 W) LED driver and operating area 120 corresponds to a second (700 mA, 150 W) LED driver, each operating area 110, 120 being indicated by dashed lines. The operating area 110 has constant maximum voltage setting of 280V and the operating area 120 has a maximum voltage setting of 210V, regardless of the applied current. Therefore, even if the second LED driver, for example, were operated at a reduced nominal current (e.g., 530 mA instead of 700 mA), the maximum voltage limit would still be 210V, and thus the second LED driver would deliver less power and be unable to utilize its full capability. To deliver the lower current of 530 mA and the same power of 150 W, the first LED driver having a maximum voltage set to 280V would have to be provided in place of the second LED driver.
Thus, there is a need in the art for a solid state lighting device driving technique in which the maximum voltage output by a driver may be varied in response to a reference current and a predetermined power limit.